The present invention relates to a solid-state image sensor, and particularly to improvement of a two-dimensional CCD image sensor operating in a field storage mode.
Two-dimensional CCD image sensors operating in a field storage mode are conventionally used in TV cameras. In the field storage mode, signal charge transfers from photodiodes are carried out for each field to achieve interlaced scanning. The transfer of signal charges in the manner mentioned is referred to as field shifting. Field shift signal charges are sequentially sent from vertical transfer registers to charge coupling portions provided for a horizontal charge transfer. The sending of signal charges in this manner is referred to as line shifting. The line-shifted signal charges are converted, by the horizontal charge transferring operation, to an output signal of the image sensor.
FIG. 1 shows the typical arrangement of the CCD image sensor's electrodes. Line shifting requires overlapped portions X, provided between electrode 12 of the vertical transfer register and electrode 15 of the horizontal transfer register. This means that a certain coupling capacitance is formed between electrode 12 and 15 via overlapped portions X. As a result, when the horizontal transfer register is operated during field shifting, the potential at electrode 12 is unfavorably modulated by horizontal driving pulses applied to electrode 15.
Further, when interlaced scanning is performed, through the horizontal transfer frequency is equal to the even number multiple of the horizontal scanning frequency is not the even number multiple of the horizontal scanning frequency in an odd field. Consequently, since the phase difference between the field shifting pulse and the horizontal driving pulse obtained in the odd field differs from that obtained in the even field, potential variations are invited in the image sensor output, thereby causing flickering in the obtained image.
According to the inventor, such flickering can be eliminated by temporarily stopping the operation of the horizontal transfer register in a CCD image sensor when field shifting is performed. Though flickering can be effectively eliminated when the operation of the horizontal transfer register is temporarily stopped for each field shift, another serious problem arises. That is, the signal level of the image sensor output (black level of the video picture) obtained when the horizontal transfer register is operated differs from that obtained when the operation of the horizontal transfer register is temporarily stopped. This is a serious problem, particularly when the sensitivity of a TV camera using the above CCD image sensor is high.
Further, if the horizontal transfer register carries out the charge transferring operation when line shifting is performed, the sharpness or resolution of the obtained image along the horizontal direction deteriorates, because charges sent from the vertical transfer register are unfavorably spread over several stages of the horizontal transfer register. To prevent such an unfavorable charge spread, the operation of the horizontal transfer register has to be stopped when line shifting is performed. Just as above, however, stoppage of the horizontal transfer register invariably gives rise to the problem of signal level differences.